The degree to which a material is able to transfer heat is called thermal conductivity. It can be defined as the time rate of transfer by conduction, through unit thickness, across unit area for unit temperature gradient. Differences in design and wall thickness of the implants used in the present study account for the variation in outcome (i.e., Straumann tissue level 4.8 implant less effected), considering the fact that all implant types used are made from the same material: grade 4 commercially pure (CP) titanium. The thermal conductivity of CP titanium is relatively low compared to, for instance, that of steel (1/4) and aluminum (1/13). On the other hand, it is approximately 60% higher than that of grade 5 titanium alloy (16.3 W · m⁻¹ · K⁻¹ for grade 4 CP titanium compared to 7.2 W · m⁻¹ · K⁻¹ for grade 5 Ti-6Al-4 V). The former material is used in several other implant brands than the ones used here [38], and as a consequence, the results cannot be extrapolated to those systems. The data from this in vitro experiment can only be generalized to the clinical situation bearing in mind some inherent limitations and assumptions. The epoxy resin used does not resemble alveolar bone, its structure, water content, and potential to cope with thermal challenges. It is unlikely that wall thickness, design, and material between different implants from the same implant type will vary because of the high degree of current precision and standardization achieved during the fabrication of implants. Consequently, only one specimen per brand was used. To correct for variation during the instrumentation of the ultrasonic device, the experiment was performed three times.

The results show that both 1. the type of implant and 2. the type of ultrasonic device (and in especially the use of coolant) affect the amount of temperature rise to the outer implant surface. Both bone level implants in particular appear to heat up the most. Without the use of coolant, the heat accumulation was much higher with the Satelec compared to the EMS device and exceeded the theoretical threshold for permanent biological damage after 10 to 15 s of continuous instrumentation (Figure 2a,c) and differed statistically significant at deltaTmax. There may be several explanations for this, but most likely, the produced energy at the point of the tip for the Satelec device was higher. Both devices were set at their lowest possible level. The present authors have not been able to verify what energy levels are actually produced. The former is presumed linear to the frequency and the deflection/amplitude of the tip. The frequency is mentioned in the product documentation (Satelec: 27 to 33 kHz and the EMS: 24 to 32 kHz) but the amplitudes differ per tip and are not disclosed. The effect of the difference in coolant spray between the two instruments is also a factor that needs considering when interpreting the data.